Modern integrated circuits are often very large and complex, including tens or even hundreds of millions of transistors, which makes them difficult and expensive to design and validate. The design and fabrication of an integrated circuit involves many different steps, many of which are commonly performed using electronic design automation (EDA) tools running on a computer. Each transistor on the integrated circuit is composed of shapes for diffusion, polysilicon, contacts, metallization, and other structures. Other structures must also be created to connect the transistors or to form other circuit elements, such as capacitors. While performing the various steps in the design process, designers must be acutely aware of the limitations of the manufacturing process. Shapes must be carefully designed to enable the implementation of the desired electronic circuit function at the resulting end of fabrication.
There are numerous structures of miniscule dimension made of various materials in close proximity to one another on each semiconductor chip. Each of those structures has a desired shape, in some cases Manhattan shapes (i.e. polygons with orthogonal edges), but any number of other shapes as well. The structures are formed using many different techniques. Photolithography is commonly used to create structures in the chip. One photolithographic technique for forming a set of structures in a particular layer of a chip is to deposit a layer of material and then to coat the layer with a photosensitive material. A light is then shined through a mask, which possesses a variety of shapes to represent the desired structures, in order to selectively expose the areas of the photo-sensitive material not covered by the shapes in the mask. The exposed areas of the photo-sensitive material, along with the layer of material below those exposed areas in some cases, are then etched away. After the unexposed photo-sensitive material is removed, the desired structures remain. Numerous other techniques can be used to form structures in the semiconductor chip, but many techniques share the use of masks, where a mask includes one or more shapes with edges which are used to create the structures on the chip either directly or indirectly. The shapes on the mask may or may not represent shapes of actual structures forms, but take a shape that is dictated by the fabrication process and the desired shapes of the structures to be formed.
As technologies have advanced, smaller and smaller dimensions, or feature sizes, have been necessary for the masks used by the lithography. Because lithography uses electromagnetic radiation (EMR), such as visible light or ultraviolet light, to selectively expose areas of the chip through a mask, if the dimensions of desired features get smaller than the wavelength of the EMR used, there can be interactions between the mask and the EMR, that impact the actual shapes of the exposed areas and make the exposed shapes different than the shapes on the mask. To accommodate this, the shape of the final exposed area can be used to generate a desired shape of a mask element that is different than the original shape.